AXISYMMETRY OF MANTLE ASPHERICAL STRUCTURES

Recent seismic tomographic models have revealed broad, low velocity anomalies in the lower mantle beneath Africa and the central Pacific that correspond to the poles of the surface geotectonic reference network. We test the hypothesis that the tomographic models are axisymmetrical with respect to axes passing near the geotectonic poles. The symmetry is apparent at the bottom of the lower mantle as well as in the transition zone in the upper mantle. This supports the idea of massive flushing of cold upper mantle material into the bottom part of the lower mantle and points to a stabilizing role of the lower mantle megaplumes on whole mantle processes.